Using a multifactorial study design, we previously showed that site and severity of infection may fundamentally alter the effects of prophylactic G-CSF in E. coli-infected rats. With intravenous E. coli challenge, granulocyte colony stimulating factor (G-CSF) pretreatment increased lethality at all bacterial dosages. However, with intrabronchial and intraperitoneal challenge, G-CSF worsened survival at intermediate bacterial dosages (LD50), but improved survival with high bacterial dosages (LD90). We have completed studies using this multifactorial design to investigate the mechanisms underlying these disparate effects of G-CSF with E. coli bacteria. Analysis of the data from these additional studies shows that the harmful or beneficial effects of G-CSF are related both to the level of intravascular infection and to G-CSF's ability to alter circulating neutrophils and microbial clearance. We are extending these studies to include S. aureus challenges to determine how type of bacteria may alter the effects of G-CSF.